Multilayer thick film circuits have been used for many years to increase circuit functionality per unit of area. Moreover, recent advances in circuit technology have placed new demands on metal conductor materials for this use. Heretofore, most of the metal conductor materials used in multiple circuits have been conventional thick film gold or copper compositions. These are comprised of finely divided particles of metal solids and inorganic binders dispersed in an inert organic medium. Such thick film materials are usually applied by screen printing in the pattern desired for the conductor.
Thick film materials of this type are very important and will continue to be so. However, when applying these thick film materials in patterns by screen printing, it is difficult to obtain fine line and space resolution. It is essential that all the screen printing variables such as screen quality, squeegee hardness, print speed, dispersion properties, etc., be most carefully controlled and constantly monitored to obtain good product yields.
An alternative approach is (1) to apply a layer of the conductive metal material to a substrate by means of dispersion in a photosensitive medium, (2) to expose the layer imagewise to actinic radiation, (3) to solvent develop the pattern to remove unexposed portions of the layer, and (4) to fire the remaining exposed portions of the pattern to remove all remaining organic materials and to sinter the inorganic materials.
Such an approach is found in Felten, U.S. Pat. No. 3,877,950 issued Apr. 15, 1975, and Felten, U.S. Pat. No. 4,598,037 issued July 1, 1986. As set forth in these patents processing involves the coating composition being (1) screen printed onto a ceramic substrate, (2) exposed imagewise to actinic radiation to effect hardening of the exposed areas of the composition, (3) organic solvent developed to remove unexposed areas of the composition, and (4) fired in air or nitrogen to effect volatilization of the organic medium and sintering of the inorganic binder.
A disadvantage of prior art photosensitive gold and copper conductor compositions is that an organic solvent is necessary to develop such material after imagewise exposure to actinic radiation, i.e., an organic solvent removes areas of the composition which have not been exposed to actinic radiation without removal of areas which have been exposed. Organic solvents often are undesirable as they may pose health and environmental hazards.
In addition, both copper and gold metals suffer from certain disadvantages. Copper conductor compositions must be fired in a non-oxidizing atmosphere in order to prevent the formation of non-conductive copper oxides. Gold conductor compositions can be fired in air, but gold is very expensive.
There exists a need, therefore for a photosensitive conductor composition using a conductive metal which can be fired in air but is less expensive than gold and which, after exposure to actinic radiation, is developable in aqueous solution.